The present invention relates to a decoding apparatus and decoding method for decoding an RF signal reproduced and read from a recording medium on which data is recorded in the form of RLL (Run Length Limited) codes, in accordance with at least one comparator level, thereby to output channel-bit data.
To transmit data or to record data on a recording medium such as a magnetic disk, an optical disk or a magneto-optical disk, the data is modulated to data that may be well transmitted or recorded. Known as one method of modulating data is block encoding. In the block encoding, a stream of data is divided into blocks (hereinafter referred to as xe2x80x9cdata wordsxe2x80x9d), each consisting of mxc3x97i bits. The data words are converted to code words in accordance with appropriate encoding rules. It should be noted that each code word is composed of (nxc3x97i) bits. The code word has a fixed length if i=1. If i is of maximum value imax=r, where r is 2 or more, the code word has a variable length. The blocks, or data words, shall be called variable-length codes (d, k; m, n; r). The value i is called xe2x80x9crestraint lengthxe2x80x9d here, and xe2x80x9crxe2x80x9d is the maximum restraint length. The value d is the smallest number of xe2x80x9c0sxe2x80x9d that may exist between two xe2x80x9c1sxe2x80x9d in a series of codes, and the value k is the largest number of xe2x80x9c0sxe2x80x9d that may exist between two xe2x80x9c1sxe2x80x9d in the series of codes.
The modulation applied to compact disks (CDs) will be described as an example of a method of modulating data. EFM (Eight-to-Fourteen Modulation) is used to record data on a CD. More precisely, an 8-bit data word is converted to a 14-bit code word (channel bits), three margin bits are added to the 14-bit code word, thus reducing the DC component in the code word (i.e., EFM word), and the code word is then recorded by means of NRZI modulation. The 8-bit data word is converted to a 14-bit code word and the margin bits are added, such that the smallest number of xe2x80x9c0sxe2x80x9d and largest number of xe2x80x9c0sxe2x80x9d may be 2 and 10, respectively. Hence, the parameter of this modulation is (2, 10; 8, 17; 1). The minimum inversion interval Tmin is 3 (=2+1)T, where T is the interval between bits in a channel-bit stream (a series of recorded waves). The maximum inversion interval Tmax is 11 (=10+1)T. The width Tw of the detection window is (m/n)xc3x97T, where T is the interval Tdata between data items in a stream of data items, and has the value of 0.47 (=8/17)T.
The minimum length dxe2x80x2 defined by identical symbols in the code word that has been NRZI-modulated is: dxe2x80x2=d+1=2+1=3. On the other hand, the maximum length kxe2x80x2 defined by identical symbols in the code word is: kxe2x80x2=k+1=10+1=11.
With the CD described above, the recording density can be increased if bits are compressed in the linear-speed direction. When the bits are compressed, the minimum bit length that corresponds to the minimum inversion interval Tmin will decrease. If the minimum bit length decreases excessively, it will be difficult to detect the bits, causing an error.
The error rate in the process of reproducing data from a disk will increase if a skew occurs, that is, if the optical pickup is inclined to the recording surface of the disk. The skew is classified into two types in accordance with the direction in which the optical axis of the pickup inclines to the disk. The first type occurs in a tangential direction in a plane parallel to the direction in which the pickup moves and perpendicular to the disk. The second type occurs in a radial direction in a plane parallel to the radial direction of the disk and perpendicular to thereto. The error rate increases in the tangential direction at a relatively early stage of the process of reproducing data. Both types of skews inevitably reduce the design margin of the system.
The distribution of errors in the minimum length defined by identical symbols was checked in the two skew directions. The errors due to the skew in the tangential direction occurred when the minimum length defined by identical symbols was short. That is, the error rate increased because the code word having a length Tmin (dxe2x80x2) was decoded to a data item having a length of Tminxe2x88x921 (dxe2x80x2xe2x88x921). It was found that many errors were made in the EFM system when 3T (i.e., minimum inversion interval Tmin) was changed to 2T, where T is the interval between bits in a series of recorded waves.
The waves reproduced have their form more distorted when the recording density is increased in the linear-speed direction or when a skew of a large angle takes place in the process of reproducing data from the disk. Thus, the error rate increases as the code word having a length Tmin (dxe2x80x2) is decoded to one having a length Tminxe2x88x921 (dxe2x80x2xe2x88x921) and the code word having a length Tminxe2x88x921 (dxe2x80x2xe2x88x922) is decoded to one having a length Tminxe2x88x921 (dxe2x80x2xe2x88x923). In other words, errors occur in the EFM system when 3T (i.e., minimum inversion interval T min), where T is the interval between bits in a series of recorded waves, is decoded to 2T, 2T is then decoded to 1T, and further 1T is decoded to 0T. Here, xe2x80x9c0Txe2x80x9d means that the output is too small or too large to cross the comparator level, or that the output cannot be detected at all.
RLL (1,7) codes, which are often used in the modulation performed in magneto-optical recording systems will be described. The parameter of an RLL code (1,7) is (1, 7; 2, 3; 2). The minimum inversion interval Tmin is 2 (=1+1)T and the maximum inversion interval Tmax is 8 (=7+1), where T is the interval between bits in a series of recorded waves.
In the process of reproducing data, using the RLL (1, 7) code, an error is made when the recording density is increased in the linear-speed direction or when a skew of a large angle occurs. That is, the minimum inversion interval Tmin, i.e., 2T, is decoded to 1T, and further 1T is decoded to 0T, where T is the interval between bits in a series of recorded waves.
The value 0T, i.e., an error that can no longer be detected, is often obtained in the case where d=1 in the RLL code (1, 7). This is inevitably because it is believed that 2T is more easily decoded to 1T and then to 0T when d=1 than when d=2, though the waves reproduced must be much distorted to be detected as an error when d=2 and 3T is therefore decoded to 2T, thence to 1T, and thence to 0T.
Some asymmetry margin is provided in the manufacture of optical disks. It is therefore necessary to take into consideration a case where the waves reproduced are asymmetric with respect to the center level.
Viterbi decoding may be employed to reduce the error rate in the process of reproducing signals. Viterbi decoding is one of decoding methods in which code errors are minimized, thereby finding the geometrically shortest way possible, discarding other ways of less likelihood. In other words, Viterbi decoding is a decoding method in which the value having the highest likelihood is searched for in a simple manner. An algorithm for compensating for the minimum inversion interval Tmin can be utilized in Viterbi decoding.
Viterbi decoding, however, is disadvantageous in that a complicated circuit of a large scale must be used to perform it. This decoding method cannot decode data without making errors, if the data has been reproduced from a recording medium that has an asymmetry margin like an optical disk. The circuit for performing Viterbi decoding should therefore be designed to cope with the asymmetry margin and is, inevitably, more complicated in structure.
A recording medium, such as an optical disk, may hardly have a sufficient skew margin. The skew margin is inadequate, particularly in the tangential direction.
It is difficult to reproduce data from a recording medium having high recording density, such as an optical disk, at with a stable minimum inversion interval Tmin. Inevitably, the error rate increases.
The inventors of the present invention has proposed in Japanese Patent Application No. 8-139264 that a run-detector be used to decrease the error rate in processing signals, by means of a circuit of a more simple structure.
In the invention of Japanese Patent Application No. 8-139264, a data decoder corrects Tmin when d=2, thereby decreasing the bit error rate. The data decoder comprises two major components, i.e., an input signal processing section and a data decoding section.
The data decoding section samples an RF signal reproduced, with a bit-clock signal supplied from the input signal processing section. The RF signal thus sampled is quantized. Meanwhile, the comparator level detected in the input signal processing section is sampled and quantized. The comparator level thus quantized is compared with the level of the RF signal quantized, by using the bit-clock signal, in a bit detecting section. If the level (amplitude) of the RF signal reproduced is equal to or higher than the comparator level, the bit detecting section outputs a channel bit data (binary signal) of logic level xe2x80x9c1.xe2x80x9d If the level of the RF signal reproduced is lower than the comparator level, the bit detecting section outputs a channel bit data (binary signal) of logic level xe2x80x9c0.xe2x80x9d
If a recording medium of high recording density, such as an optical disk, has high asymmetry, the bit detecting section outputs will output a channel bit data (binary signal) of logic level xe2x80x9c0 xe2x80x9d even if when the amplitude of the RF signal is equal to the comparator level. Consequently, the accuracy of the decoding performed by the data decoding section may decrease in some cases.
The present invention has been made in view of the foregoing. The object of the invention is to provide a decoding apparatus and decoding method that can decode a reproduced RF signal with a high accuracy even if the level (amplitude) of the RF signal is equal to the comparator level at the time of detecting a bit.
A data-decoding apparatus according to this invention compares the level of data with a threshold level, thereby to generating a decoded bit. The apparatus comprises decoded-bit determining means, which determines whether the decoded bit is xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d from the levels of two data items respectively preceding and following the data when the data has a level equal to the threshold level.
A data-decoding method according to the present invention is designed to decode a bit by comparing the level of input data with a threshold level, thereby generating a decoded bit. In the method, it is determined whether the decoded bit is xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d from the levels of two data items respectively preceding and following the input data, when the level of the input data is equal to the threshold level.
In the present invention, the accuracy of decoding the reproduced RF signal can be increased even if the level (amplitude) of the RF signal is equal to the comparator level at the time of detecting a bit.